cya, crp, rho and nus genes modulate the expression of a wide variety of bacterial and bacteriophage genes or operons. In order to understand the regulatory processes, we are studying the structure, expression and activity of these genes. We have cloned these genes and characterized them (a) physically by restriction analysis and electron microscopy, and (b) biologically by isolation of mutants, mainly ambers and insertions. The protein products of the cya, crp and rho genes are autogenously regulated. The autogenous regulation of the rho gene is at th level of transcription. Cyclic AMP modulates rho gene expression in two opposite ways: it enhances rho transcription but inhibits rho translation. We have isolated new crp mutants which suppress the adenylate cyclase defect of a mutant cell.